1. Field of the Invention
The present invention relates to e signal transmission system for transmitting signals between two transceivers simultaneously and bidirectionally.
2. Description of the Related Art
A conventional system for transmitting signals between two ports simultaneously and bidirectionally is described in xe2x80x9c1 Gb/s Current-Mode Bidirectional I/O bufferxe2x80x9d, 1997 Symposium on VSLI Circuits, Digest of Technical Papers, pp. 121-122.
FIG. 6 illustrates the configuration of the conventional system as described in this article.
The transmitter side of each port turns its switch ON/OFF according to a logical level of a signal IN1 or IN2, thus controlling the current flowing from a constant current circuit to a transmission line 30.
A signal is transmitted from the port of a chip CHIP1 to the port of a chip CHIP2 via the transmission line 30 while another signal is simultaneously transmitted from the port of the chip CHIP2 to the port of the chip CHIP1 via the transmission line 30.
Table 1 below illustrates the operation of the conventional system shown in FIG. 6.
As shown in Table 1, the receiver side of each port detects the voltage at a node O1 and the voltage at a node O2 so as to determine the logical level of the signal transmitted from the transmitter side of each port.
The conventional system described above has the following problems (1) and (2).
(1) The conventional system has a substantial power consumption. For example, when each port transmits the signal IN1 or IN2 having a logical level of xe2x80x9c1xe2x80x9d (xe2x80x9chighxe2x80x9d), the total current of 4*I flowing out of four constant current sources is consumed. Herein, xe2x80x9cIxe2x80x9d denotes a current flowing out of one constant current source. Particularly, the problem of having a substantial power consumption is more pronounced as the number of ports that are simultaneously operated is increased.
(2) The direction along the transmission line 30 in which a current I3 flows changes according to the logical level of the signal being transmitted. When the direction of the current I3 changes, it is difficult to suppress the electromagnetic radiation from the transmission line 30. Moreover, when the direction of the current I3 changes, it may be necessary to control the current to flow from a position where the power source potential is low to another position where the power source potential is highs This is because a voltage drop occurs due to the resistance of the power source line in a case where the power source potential is provided to each port via the power source line. Controlling a current to flow from a position where the power source potential is low to another position where the power source potential in high is very difficult because it is against the law of nature.
According to one aspect of this invention, there is provided a bidirectional signal transmission system including a first transceiver, a second transceiver, and a transmission line connecting the first transceiver and the second transceiver with each other, in which the first transceiver transmits a first signal to the second transceiver while the second transceiver simultaneously transmits a second signal to the first transceiver. The first transceiver includes: a first constant current circuit out of which a current flows, the current having a level which changes according to a logical level of the first signal; and a first current difference compensation circuit for compensating for a difference between the current flowing out of the first constant current circuit and a current flowing along the transmission line. The second transceiver includes: a second constant current circuit into which a current flows, the current having a level which changes according to a logical level of the second signal; and a second current difference compensation circuit for compensating for a difference between the current flowing into the second constant current circuit and the current flowing along the transmission line.
In one embodiment of the invention, a difference between the current flowing out of the first constant current circuit and the current flowing into the first current difference compensation circuit is greater than a difference between the current flowing out of the second current difference compensation circuit and the current flowing into the second constant current circuit.
In one embodiment of the invention, a power source potential is provided to the first transceiver and the second transceiver via a power source line, and a direction in which a voltage decreases along the power source line coincides with a direction in which the current flows along the transmission line.
In one embodiment of the invention, the bidirectional signal transmission system further includes a current consumption stabilization circuit for constantly maintaining a current consumption in the bidirectional signal transmission system independently of the logical level of the first signal and the logical level of the second signal.
According to still another aspect of this invention, there is provided a bidirectional signal transmission system for signal transmission between a first transceiver and a second transceiver. The bidirectional signal transmission system includes a plurality of transmission units. Each of the transmission units includes a transmission line for connecting the first transceiver and the second transceiver with each other, transmits a first signal from the first transceiver to the second transceiver while simultaneously transmitting a second signal from the second transceiver to the first transceiver, and generates onto the transmission line a current which flows in a predetermined direction for the transmission unit irrespective of a value of the first signal and a value of the second signal, the predetermined direction being one of a first direction from the first transceiver to the second transceiver and a second direction from the second transceiver to the first transceiver. Among the plurality of transmission units included in the bidirectional signal transmission system, the number of transmission units which generate a current which flows in the first direction is substantially equal to the number of transmission units which generate a current which flows in the second direction.
In one embodiment of the invention, each of the transmission units further includes a first constant current circuit out of which a current flows, the current having a level which changes according to a logical level of the first signal; a first current difference compensation circuit for compensating for a difference between the current flowing out of the first constant current circuit and a current flowing along the transmission line; a second constant current circuit into which a current flows, the current having a level which changes according to a logical level of the second signal; and a second current difference compensation circuit for compensating for a difference between the current flowing into the second constant current circuit and the current flowing along the transmission line. Each of the first constant current circuit and the first current difference compensation circuit is connected to an end of the transmission line on the side of the first transceiver. Each of the second constant current circuit and the second current difference compensation circuit is connected to another end of the transmission line on the side of the second transceiver.
Thus, the invention described herein makes possible the advantages of (1) providing a bidirectional signal transmission system having a low power consumption; and (2) providing a bidirectional signal transmission system in which the direction of a current flowing along a transmission line does not change according to the logical level of the signal being transmitted.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.